Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/1999
Publication Date: N/A
Interpretive Summary: Heavy metal and radioactive metal (e.g. U, Pb, Zn, and Cd) contamination of soils poses serious problems to both human health and agriculture in the U.S. Current engineering-based technologies used to remediate soils (e.g., removal of top soil for storage in landfills) are quite costly, and often dramatically disturb the landscape. Recently, there has been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil and concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. However, little is currently understood about the traits that would make certain plant species the best candidates for phytoremediation of contaminated soils. There are a number of Department of Energy sites in the U.S. that are contaminated with uranium (U), as a result of weapons research, nuclear reactor operation and nuclear fuel production. The possibility that plants could be used to clean up U-contaminated soils is an intriguing possibility. We had previously shown that one particular U species in the soil, UO22+, is translocated much better than other forms of U to the plant shoot. In the current study we show that citrate effectively solubilizes soil U as a complex with UO22+, and renders it available for uptake into the plant. We also determined the conditions which optimize U solubilization in the soil, and also show this solubilization is transient. Four days after addition of citrate to the U-contaminated soil, the U solubility is back to control levels due to citrate decomposition. Thus, it appears that addition of citrate to U contaminated soil may be an effective strategy to induce U accumulation in plant shoots, in a way that will minimize movement of soil U into the groundwater.
Technical Abstract: The role of acidification and chelating agents in the solubilization of uranium (U) from contaminated soil was examined in a series of experiments. Soil acidification and the addition of chelating agents were the two methods compared initially. The results indicated that the addition of citric acid solubilized more U than acidification or the other amendments tested. This increase in U solubility was, however, transitory. A subsequent experiment indicated that citrate concentration had a more dramatic effect on U solubility than did acidification. The greatest soluble U concentration during this experiment (775 mg kg-1 soil, or ~85% of the total U) was observed after 24 hours in the presence of 20 millimoles citrate kg-1 soil at pH 5. The persistence of U solubility over the 96-hour experimental period was primarily a function of pH and citrate degradation. In a separate experiment, in which citric acid rather than citrate was added to contaminated soil, the soluble U concentrations observed were generally lower than those observed in the presence of citrate. Citric acid decreased soil pH to values <3.6, and solubilized higher concentrations of Al and Fe than observed in the presence of citrate. Since the maximum solubilization of U was observed at pH 5, the implication of these results is that a combined approach, utilizing both soil acidification and citric acid addition, may be necessary in order to maximize the phytoextraction of U from soils with a pH>6.0.